The vast reserves of coal in this country and throughout the world, have prompted and continue to prompt considerable interest and investigation into economical processes for the transformation of coal solids into liquid products that can be upgraded to provide synthetic petroleum fractions.
U.S. Pat. No. 4,202,757 (the '757 patent) incorporated herein by reference, describes a process believed to represent a major break through in coal liquification technology, largely due to the fact that the process is designed to be carried out under normal atmospheric pressure. An improvement in this process has now been discovered whereby the components of the essential superacid system used in the process can be quickly and economically separated from the liquid hydrocarbon products, recovered and reused.
The process which is fully described in U.S. Pat. No. 4,202,757 rapidly converts coal as well as other fossil fuel sources such as oil shale or tar sands to valuable liquid hydrocarbon products. The '757 process is designed to operate with a low energy input at relatively low temperatures and atmospheric pressure, and thus, is far more economical than other processes presently known and used in synthetic petroleum technology. A potential drawback of the '757 process is that the recovery of the super-acid system is low. The present invention provides a novel process for the retrieval of the super-acid system from the hydrocarbons which make the overall process more economical.
In the process of the '757 patent, pulverized coal is initially reacted with acids, such as hydrogen halides, hydrogen pseudohalides and sulphonates in accordance with the following reaction scheme: ##STR1## wherein R represents unsaturated bonds in the coal and HX is the general formula of the particular acid used. A critical parameter in choosing a suitable acid (HX) is that the acid molecules must be capable of donating a negative ligand to a strong Lewis acid in order to form carbonium ions. [See reaction (4) and related discussion, infra.] Suitable acids for the initial phase of the coal liquification process include hydrogen chloride, chlorosulphonic acid, hydrogen fluoride, fluorosulphonic acid, hydrogen bromide, hydrogen iodide, sulphuric acid. Combinations of such acids are also contemplated for use in the initial reaction, however, hydrogen chloride and hydrogen fluoride are the preferred acids for use in this first phase of the coal liquification process. The specific details of this first phase addition reaction are disclosed in U.S. Pat. No. 4,202,757.
In the second phase of the '757 coal liquification process, the RHX (slurry) from reaction (1) is reacted with a Lewis acid, halide-ion-acceptor system, a.k.a. super-acid system, e.g. antimony pentafluoride in hydrogen fluoride. Group V halides are preferred for use in said system and include, inter alia, antimony pentachloride, antimony pentafluoride, bismuth pentafluoride, arsenic pentafluoride, phosphorous pentafluoride and phosphorous pentachloride. The bromides and iodides of the Group V elements are not as efficient in their Lewis acid properties as the acid outlined above and not all of them are known to exist in the pentavalent state. Chemical compounds wherein there are some fluorines and chlorines on the same atoms are also suitable, e.g. SbCl.sub.2 F.sub.3 or SbCl.sub.3 F.sub.2. A general formula for the suitable Group V halide compounds is: EQU MX.sub.n Y.sub.m ( 2)
wherein M is a Group V atom in the .sup.+ 5 oxidation state and X and Y are halogens which can be the same (SbF.sub.5) or different (SbClF.sub.4) and the sum of n and m equal 5. As a further criterion, the compound must have sufficient Lewis acidity to effect the following reaction: ##STR2## Suitable acids for use in the super-acid system include hydrogen fluoride, hydrogen chloride, chlorosulphonic acid and fluorosulphonic acid. The equivalent bromo and iodo acids are also suitable, although not preferred due to their lower reactivities and the undesirable problem of oxidizing the bromide and iodide ions to their elemental state. While many effective super-acid systems will be apparent to those skilled in the art, representative systems include combinations of the acids such as HF and HSO.sub.3 F with SbF.sub.5. The metal pentahalides can also be combined, for instance SbF.sub.5 and BiF.sub.5 (thus trinary, quaternary or even higher orders of systems are feasible). Other systems may include halide ion-acceptors such as pentaphenylbismuth (C.sub.6 H.sub.5).sub.5 Bi or phenyl tetrachloroantimony C.sub.6 H.sub.5 SbCl.sub.4. Furthermore, super-acid systems may be solid rather than liquid, such as SbF.sub.5 with TiO.sub.2 (titanium dioxide) or SbF.sub.5 with SiO.sub.2 (silicon dioxide).
The reactions in the second phase of the '757 process are believed to proceed in accordance with the following reaction scheme and are discussed in detail in the '757 patent specification: EQU RHX+2SbX.sub.5 +2HX.fwdarw.RH.sup.+ +2SbX.sub.6 -+H.sub.2 X.sup.+( 4) EQU RH.sup.+ +R.sup.1 H.fwdarw.RH.sub.2 +R.sup.1 ( 5) EQU 2SbX.sub.6.sup.- +2H.sub.2 X.sup.+ .fwdarw.4HX+2SbX.sub.5 ( 6)
As a result of the reaction of RHX with the super-acid system (3) (4), carbonium ions are formed which when reacted with a suitable hydrogen donor source yield commercially valuable, liquified, hydrogenated products (5). These liquid hydrocarbon products must, thereafter, be separated from any remaining solids as well as separated from the components of the super-acid system, which can be recycled to make the process continuous. To date, conventional techniques for separating the super-acid system from the hydrocarbons have proved unsatisfactory. The invention of the present application provides a novel super-acid separation step which operates compatibly with the basic '757 liquification/gasification process.